In the commercial manufacture of cheese, fermentation of the cheese milk is initiated by inoculating the cheese milk with a culture of selected bacteria. Such a culture of bacteria is known as a cheese starter.
To achieve a sufficient bacterial activity and quantity in the milk within a reasonable time, it is usual to cultivate the selected strain or strains of bacteria through several stages of increasing volume in a sterile medium such as sterilised milk to a "mother culture" stage. There follows a culture known as a "bulk starter" which forms the inoculum for the manufacturing milk. Bulk starters are usually cultivated within the cheese factory, and cannot be stored for long periods. Because the starter is transferred through several stages, the manufacture of bulk starters is not only time consuming but carries the risk of infection, not least by bacteriosphage. Another problem inherent in bulk starters is that the activity of the culture may be difficult to control with precision.
To overcome some of the problems inherent in the bulk starter method, it has been proposed to produce concentrated starters in which the bacteria have been harvested from the medium in which they were grown. Such concentrates have a sufficiently high activity that they may be added directly to the cheese vat replacing many times, typically fifty, their own volume of bulk starter. It has been proposed to manufacture such concentrated starters outside the cheese factory and away from the factory bacteriophage population, and to store them at cryogenic temperatures until required for use. The present invention relates to frozen, concentrated cheese starters.
Cheese starters may be either "single strain" cultures or "mixed" cultures. A mixed culture may not only contain several strains of the same species, but may contain several species or even several genera. The choice between a single strain culture and a mixed culture depends both on the variety of cheese being produced, and on tradition. It is principally in Australia and New Zealand that single strains have been used commercially, and even in the former country, mixed cultures have often been preferred. In the United States of America, virtually all cheese is made from mixed cultures. The commercially available mixed cultures are identified by code number, and the component strains and species are not, as a rule, characterised.
When mixed cultures are grown in standard media such as milk or whey, occasional bacteriophage infection is unavoidable. Because the susceptibility to any given bacteriophage varies widely from one microorganism or strain of micro-organism to another, the relative strength of the components can vary grossly as a result of such infection. Even in the absence of infection, the components of a mixed culture may not always attain equilibrium, and strain dominance and disturbance of proportions may occur as a result of differential growth rates, excretion of antibiotic agents, and other such effects. The relative proportions of the components are also significantly affected by changes in the composition of the medium, including seasonal changes in milk composition.
The susceptibility to bacteriophage attack can be reduced by growing the cultures in "bacteriophage inhibitory media", but strain imbalance due to other causes is found to be variably accentuated in those media.
Variations in the proportions of the various components in a mixed culture cause discernible variations in the properties of the cheese. Once a bacteriophage infection has taken place, it is extremely difficult to get rid of the offending bacteriophage from the factory environment because of the lack of characterisation of the microorganisms in the culture or available alternative mixed cultures.
By using single strain cultures, many of the disadvantages inherent in mixed cultures may be avoided. For example, if a single strain culture is attacked by a given bacteriophage, a different strain of the same species which has been characterized as having no appreciable susceptibility to the same bacteriophage may be substituted. Accordingly, to guard against the risk of bacteriophage infection, it has been proposed to employ several "phage-unrelated", single-strain cultures in rotation. Even so, bacteriophage for one strain can sometimes undergo mutation to increase its virulence towards the other strains.
The consequences of bacteriophage attack on a single-strain starter culture or in a single-strain cheese vat are much more drastic, since so many of the population can be destroyed as to lead to complete failure of acid production. Another major drawback of single-strain cultures is in the variation of the taste and texture of the product produced by different strains. Such an inconsistent product does not find favour with consumers. Furthermore, the different single strains have differing temperature susceptibilities, which means that, for optimum performance, the cheesemaking procedure needs to be varied as the strain is changed.